WHAT ARE SUBTERRANEAN AMPHIPODS ?
Gammaridean amphipods are important components of aquatic subterranean faunas in many parts of the world. They are among the most abundant, widespread, and taxonomically diverse organisms found in subterranean groundwater systems. Taxonomic diversity is significant at the family, genus and species levels, and species richness is especially remarkable in several exclusively subterranean genera such as Niphargus and Stygobromus, which contain over 100 described species each. The vast majority of subterranean amphipods are stygobites (also called stygobionts, or sometimes troglobites if they are associated with caves per se). By definition stygobites are species generally restricted to subterranean groundwater habitats and characterized morphologically by loss or severe reduction of eyes and pigment, often accompanied by attenuation of the body and/or appendages (especially the antennae and last two pairs of pereopods). This rather unusual morphology is called troglomorphy. However, some species occur simultaneously in epigean (surface) and hypogean waters and do not exhibit the advanced level of morphological modification or troglomorphy that is typical of most stygobites. They are called stygophiles or sometimes troglophiles. A few rare subterranean species in the family Talitridae are not strictly aquatic but are otherwise restricted to damp cave passages with high humidity.
On a global scale, approximately 36 families (or family groups) and 166 genera of amphipods contain sytgobites, and many of these genera and even some of the families are troglomorphic and known only from subterranean waters.
The following families contain the majority of subterranean amphipod species (number of genera with stygobionts in parentheses): Bogidiellidae (33), Crangonyctidae (5), Gammaridae (17), Hadziidae (51), Ingolfiellidae (3), Niphargidae (8), and Pseudoniphargids (family group) (2).
Most of the remaining subterranean species belong to the families Metacrangonyctidae (2), Paramelitidae (10), Pseudocrangonyctidae (2), Salentinellidae (4), and Sternophysingidae (1); while still fewer are in the families Allocrangonyctidae (1), Hyalidae (1), Metaingolfiellidae (1), Paracrangonyctidae (2), and Talitridae (3).
The majority of subterranean amphipods live in freshwater, but some are found in brackish and even fully marine water. Subterranean amphipods, like a number of other crustaceans that live in hypogean environments, such as thermosbaenaceans, isopods, shrimps, occur in a relatively wide variety of groundwater habitats . The number of described stygobitic species is currently around 882 and continues to increase rapidly with new discoveries.
Subterranean amphipods are interesting biogeographically because of their taxonomic diversity, limited dispersal ability, and significant geographic isolation and restriction to groundwater aquifers, which is reflected by the large number of local endemics. Moreover, many taxa probably represent old phylogenetic lineages that have persisted in well-buffered groundwater habitats for long periods of time and are genuine relicts, both phylogenetic and distributional.
Generalized distribution of approximately 150 species of Stygobromus (including undescribed species) in North America (excluding Alaska) is indicated by darkly shaded areas. Lightly shaded areas were unglaciated during the Pleistocene, whereas the unshaded part was glaciated. (See also "What are subterranean amphipods" elsewhere on this website.)
Geographic distribution of Stygobromus in Eurasia. Localities for three species are indicated by closed circles.
Geographic distribution of Stygobromus in British Columbia and Alaska. Localities for three species (two undescribed) are indicated by closed circles.
It is commonly
believed that stygobitic amphipods evolved from epigean
ancestors, which had developed some degree of pre-adaptation for
a hypogean life style prior to invasion of subterranean waters.
Putative ancestors of stygobites presumably gained access to and
colonized hypogean groundwaters from both freshwater and marine
environments. However, there is good evidence to suggest that stygobites can also evolve from other
stygobiotes through
speciation in subterranean waters. Subterranean habitats provide
many opportunities for the geographic isolation of local
populations, which will enhance speciation and ultimately result
in the evolution of new subterranean taxa.
The geographic distribution of virtually all
subterranean amphipods falls into one of three broad
biogeographic patterns, which, in turn, apparently reflect modes
of origin. These patterns include:
(1) Old Freshwater Distribution, which is exhibited by freshwater stygobites of inland (usually continental) groundwaters having strong phylogenetic affinities with other freshwater taxa and presumably derived from freshwater ancestors by invasion from surface waters. This pattern is strongly exemplified by stygobitic crangonyctids.
Example of an Old Freshwater Distribution pattern
(2) Marine Relict Distribution, which is exhibited by freshwater stygobiont inhabitants of continental or insular groundwaters having strong morphological affinities with marine taxa and presumably derived from marine ancestors by stranding following the regression of marine embayments. Stygobitic hadziids provide numerous examples for this pattern.
Example of a Marine Relict Distribution pattern
(3) Marine-brackish Water Distribution, which is exhibited by marine or brackish water stygobitic inhabitants of interstitial or anchialine waters in coastal and insular areas having strong morphological affinities with other marine taxa and presumably derived from marine ancestors by invasion from shallow epigean waters. A number of stygobitic hadziids also provide many good examples for this pattern.
Example of a Marine-brackish Distribution pattern. Generalized distribution track for subterranean amphipods and four other monophyletic crustacean groups in the Caribbean region.